1. Field of the Invention
The present invention relates to a multilevel QAM (quadrature amplitude modulation) demodulation circuit, and more specifically to a carrier reproduction control loop circuit for a local carrier oscillator for demodulation.
2. Description of Related Art
A multilevel QAM transmission system, which is one of digital transmission systems, has been well known in the field of a microwave radio communication from the past. Recently, attention has been paid to a bi-directional TV, an internet distribution using a cable modem, etc., and the multilevel QAM transmission system is being studied as the technology for transferring a large amount of digital data in the field of a cable transmission.
In the multilevel QAM transmission system in the field of the radio communication, a major problem is how to compensate for transmission distortion caused by a channel characteristics variation attributable to various factors including meteorological conditions. However, since communication is based on a fixed channel in one-to-one relation, a problem of the transmission distortion caused by reflection and another problem of the channel switch-over could not have become a major problem.
In a cable transmission, however, the transmission channel characteristics depends upon a construction of the transmission channel, and therefore, the degree of transmission distortion does not change almost. On the other hand, since the cable transmission is fundamentally based on a one-to-multipoint transmission, the transmission distortion is caused by reflection in a transmission cable. In addition, since it is necessary to conduct the transmission while switching over from one channel to another, a quickness of synchronism detection after a frequently occurring channel switch-over becomes an important problem. In the other words, the problems which were not the major problems in the radio transmission, have becomes the major problems in the cable transmission.
On the other hand, in order to reduce the cost of apparatuses, research is required to construct a roll-off filter into a digital LSI circuit, in place of a conventional analog SAW (surface acoustic wave) filter.
Referring to FIG. 1, there is shown a block diagram illustrating a construction of a conventional quadrature demodulation circuit used in a conventional multilevel QAM demodulation system and using a digital roll-off filter.
As shown in FIG. 1, the conventional quadrature demodulation circuit is so constructed that, an intermediate frequency signal (abbreviated to an "IF signal" hereinafter) (multilevel QAM signal) supplied to a signal input terminal 1, is divided into two signals, namely, a first IF signal and a second IF signal. The first IF signal is supplied to a mixer 3 where the input signal is multiplied with an in-phase (equiphase or same-phase) local carrier which is reproduced in a voltage controlled local carrier oscillator (abbreviated to a "VCO" hereinafter) 5, and the mixed signal is supplied through a low pass filter (abbreviated to a "LPF") 20 for the purpose of eliminating surplus high frequency components, so that an in-phase signal I is obtained. On the other hand, the second IF signal is supplied to a mixer 4 where the second IF signal is multiplied with a quadrature local carrier which is obtained by supplying the local carrier reproduced in the VCO 5, into a 90.degree. phase shifter 6, and the output of the mixer 4 is supplied to the a low pass filter 21 for the purpose of eliminating surplus high frequency components, so that a quadrature signal Q is obtained.
The in-phase signal I and the quadrature signal Q are supplied to an analog to digital (A/D) converters 7 and 8, respectively, so that the analog in-phase signal I and the analog quadrature signal Q are converted into a digital in-phase signal ID and a digital quadrature signal QD, respectively. The digital in-phase signal ID and the digital quadrature signal QD are supplied through roll-off filters 9 and 10, respectively, to a waveform equalizer 12, in which these signals are subjected to a waveform equalizing processing, and then, converted into an in-phase demodulation signal IMD and a quadrature demodulation signal QMD, respectively.
The in-phase demodulation signal IMD and the quadrature demodulation signal QMD, obtained from the waveform equalizer 12, are converted by a code conversion circuit 16, and then, an error detection/correction is conducted by an error detection/correction circuit 17, so that an error corrected signal is outputted through a data output terminal 18.
Furthermore, the in-phase demodulation signal IMD and the quadrature demodulation signal QMD are fed back to a carrier phase error detection circuit 11, in which a carrier phase error is detected, and an error signal is fed back through a digital to analog (D/A) converter 19 to the VCO 5, so that the VCO 5 reproduces the local carrier having a correct frequency and phase. Thus, a control loop is constituted.
Here, the conventional carrier phase error detection is conducted by using the outputs of the waveform equalizer 12. The reason for this is that: If the carrier phase error detection was conducted by using the signals which have not yet been subjected to the waveform equalizing processing, since the signals includes the transmission distortion, the carrier phase error detection circuit 11 mis-discriminates the transmission distortion as the phase error to change the control voltage supplied to the VCO 5, with the result that the local carrier rather becomes unstable.
However, when the roll-off filters 9 and 10, which were constructed of the analog SAW filter in the prior art, are constituted of a digital circuit, the roll-off filters 9 and 10 are put after the A/D converters 7 and 8 as shown in FIG. 1. In this case, in order to obtain a desired characteristics, the roll-off filter must have a number of taps, with the result that the carrier recovery or reproduction loop delay becomes large.
In order to realize a stable carrier reproduction, for example, Japanese Patent Application Pre-examination Publication No. JP-A-02-150145, (an English abstract of which is available from the Japanese Patent Office, and the content of the English abstract of JP-A-02-150145 is incorporated by reference in its entirety into this application) proposes a multilevel QAM transmission system for a multiplex digital radio transmission, which is configured to adaptively switch over one signal channel to another in accordance with the degree of stability in frequency.
Referring to FIG. 2, there is shown a block diagram illustrating the construction of the quadrature demodulation circuit used in the multilevel QAM transmission system proposed by JP-A-02-150145. In FIG. 2, elements similar or corresponding to those shown in FIG. 1 are given the same Reference Numerals, and explanation thereof will be omitted.
As seen from comparison between FIGS. 1 and 2, the carrier phase error detection circuit 11 shown in FIG. 1 is replaced with a carrier phase error detection circuit 30, which comprises a control circuit 301 receiving a pair of input signals of the waveform equalizer 12 for generating a phase error signal, another control circuit 302 receiving the in-phase demodulation signal IMD and the quadrature demodulation signal QMD outputted from the waveform equalizer 12, for generating a phase error signal, a selector 303 receiving the phase error signal generated by the control circuit 301 and the phase error signal generated by the control circuit 301, a comparator 304 comparing the phase error signal generated by the control circuit 301 with the phase error signal generated by the control circuit 301, for generating a control signal for the selector 303, and a loop filter 305 receiving an output of the selector 303 for filtering the received signal to output the filtered signal to the D/A converter 19. In addition, the error detection/correction circuit 17 is not provided in the circuit shown in FIG. 2, but this is not an essential difference.
In the circuit shown in FIG. 2, the phase error signal (or control result) obtained from the input signals of the waveform equalizer 12 is compared with the phase error signal (or control result) obtained from the output signals of the waveform equalizer 12. Here, it is considered that, if both are consistent, it is called a "stationary time", and if both are not consistent, it is called an "unstationary time", namely, in a condition in which since the transmission distortion occurs, the waveform equalizing processing is conducted in the waveform equalizer 12. Therefore, at the stationary time, a control loop having a small delay is constituted by using the input signals of the waveform equalizer 12, and at the unstationary time, a control loop having a large delay is constituted by using the output signals of the waveform equalizer 12. As a result, since the control loop having the small delay is used at the stationary time, it is possible to provide a circuit having a fast response to frequency variation.
In the cable transmission, however, the transmission channel characteristics is determined by fixed factors including the structure of the transmission channel, not by variable factors such as meteorological conditions. Therefore, the transmission channel of a bad condition remains bad indefinitely, so that the control loop having the large delay is constituted always.
In addition, when the channel is switched over, the control loop becomes the control loop having the large delay, and therefore, the delay time is not improved until the synchronism becomes stable.
Furthermore, since the roll-off filter is included both in the control loop having the large delay and in the control loop having the small delay, the loop delay cannot be shortened.
As mentioned above, in the conventional quadrature demodulation circuit, since the roll-off filter and the waveform equalizer are included in the carrier reproduction loop, the response to the frequency variation is low. Therefore, when the channel is switched over, it is not possible to quickly responds to the frequency change, and therefore, a substantial time is required, until the synchronism becomes stable.
The reason for this is considered as follows: In the conventional quadrature demodulation circuit, if the roll-off filter, which was constructed of the analog SAW filter in the prior art, is digitized and internally provided in a demodulation IC for the purpose of reduce the cost, it is necessary to provide a number of taps in order to obtain a desired characteristics, with the result that the loop delay becomes large.
In the multilevel QAM transmission system, the waveform equalizer is indispensable to compensate for the waveform distortion. In applications including CATV (cable television), the waveform equalizer inevitably has an increased number of taps, and therefore, the loop delay becomes further large.